This invention relates generally to the recovering of the free metal entrained in dross or skimmings obtained from the production of aluminum or aluminum based alloys.
In the course of conventional aluminum melting operations, oxides, nitrides and other non-metallic impurities accumulate on the surface of the molten metal. Prior to tapping of the molten metal these non-metallics are removed or skimmed from the surface of the melt. Substantial quantities of aluminum metal will be unavoidably entrained with the non-metallics and also be removed with the non-metallics. This mixture of non-metallics, free aluminum and aluminum alloy is termed aluminous dross or skim. For convenience, this mixture of non-metallics and free aluminum or aluminum alloy will hereinafter, in the specification and the appended claims, be referred to as dross.
As stated above, the dross derived from aluminous metal melt unavoidably contains a subtantial proportion of free metal and/or alloy as a result of the usual stirring of the melt and raking off of the floating material. During the raking, skimming and removal of the dross from the top of the metal melt, the dross becomes compresed into pasty mud-like masses. These masses of dross, when removed from the furnace will vary from small lumps on the order to one inch in dimension and smaller to large lumps approaching one foot in dimension for example. The amount of free metal and/or alloy in the dross may vary from 30% to 95% by weight depending upon a number of factors, such as the composition of an alloy being melted, the melting procedure followed, and the care with which the dross is skimmed or raked from the melt. If a batch of hot dross removed from a melt is allowed to stand, some free metal will accumulate at the bottom of the mass, but the larger part of the free metal will remain intimately mixed with non-metallics in the form of globules or small particles and will not readily separate from the non-metallic portion. Also, upon being exposed to the atmosphere, the hot dross may begin to react with the air, if the reaction has not already started within the furnace; and if the reaction is not stopped, a large part of the available metal will be lost. The separation of the free metal from the non-metallic portion of the dross has been a difficult problem. Several methods used or proposed for effecting separation are mentioned below.
In one method, the dross is cooled to room temperature as quickly as possible, screened, crushed as in a ball mill, and then screened again. By this mechanical means of separation, the coarser metal particles can be separated and recovered. However, the bulk of the free metal in the dross is in the form of small particles which heretofore has made recovery by mechanical means unattractive.
In another method the hot dross is stirred into a heel of molten aluminum or aluminum alloy. This method is not efficient because in agitating the dross in the molten metal heel, nearly as much metal is beaten into the dross as is removed.
In still another process, after skimming of the dross, it is fed without further preparation into recovery furnaces, fluxed with salt for example, and metal recoveries are obtained. This process is not efficient because of low metal recoveries, high energy costs per pound of metal recovered and a serious disposal problem of the resulting slag which, because of its salt content, has become environmentally unacceptable.
Common salt, i.e. sodium chloride, is employed in this process due to the low cost of the material. More expensive salt type fluxes may be used to increase metal recovery to a limited extent, but then such additional cost for flux offsets the increased effectiveness related thereto. However, while the cost of sodium chloride is low, the recovery of molten metal involved has also been very low. These low recoveries are due to the fact that common salt fails to efficiently attack the oxide coatings on the small droplets of aluminum material entrapped in the aluminous dross. The use of common salt has a further disadvantage in that substantial heat is required to melt the salt, its melting point being at a temperature of approximately 800.degree. C. (approximately 1480.degree. F.). If satisfactory melting is to be made possible, the salt bath must be heated at a temperature substantially above its melting point in order to have sufficient fluidity, and it must be kept at this temperature during the introduction of the aluminous dross and during melting down. For example, where the salt melts at 1480.degree. F., the bath would have to be heated to a temperature on the order of 80.degree. F. higher, or about 1560.degree. F. in this instance. For melting and treating aluminum, the maximum temperature permissible for best results is approximately 1500.degree. F. Above this temperature deterioration of the quality of the metal and undesirable fumes result. Furthermore, when the aluminous dross-salt flux mixture is heated to 1560.degree. F., it possesses a considerable dissolving power for all metals which come into consideration as impurities. In addition, the hot common salt melt strongly attacks furance lining.
According to a more recent process, hot dross, which may be either the dross as removed from the melting furnace or cold dross which has been reheated, is placed in an inclined rotatable drum open to the air, and the dross is rotated therein for a short period of time. If the dross is not already burning when introduced into the drum, ignition is started by the addition of suitable salts. In this process, a portion of the finely divided free metal is consumed in reacting with the air to provide the heat essential for raising the temperature of the mass. Consequently, the recovery of metals is not as high as desired. Metal recoveries on the order of 65% to 70% of the available metal have been achieved by this method, but on the average the recovery has been found to be below 60%. In addition, it is difficult to control the furnace temperature when employing this process, and generally the temperature is well above 1500.degree. F. with the attendant disadvantages thereof.
With respect to all of the prior art methods for recovering aluminum metal, it is to be understood that dross, as generally referred to hereinabove, exists in particles and chunks of material of various sizes. A represenative sample of dross, after initial conventional milling and screening preparation, may contain the following size ranges, aluminum content and recoverable aluminum using the better of the recovery methods described heretofore:
______________________________________ Approxi- mate Fur- Approxi- nace Re- mate Fur- covery of Approxi- nace Re- Metallic as % of mate covery of % of En- Dross Metallic Metallic as trained Size Range Load Content % of Dross Metallics ______________________________________ +1 inch 10% 90% 81% 90% -1 inch +.5 inch 12% 85% 68% 80% -.5 inch +.25 inch 13% 80% 56% 70% -.25 inch +.10 inch 15% 75% 38% 50% -.10 inch 50% 45% N/A N/A ______________________________________
The size ranges of -0.10 inch and down, if subjected to the heat of the furance, would be consumed in the heat of the furnace and lost. Therefore, these size ranges are generally screened off and sold as low percentage metallic content aluminum oxide dust.
Using a representative sample of dross, screening off the -0.10 inch size range, and using the balance for furnace recovery, the following approximate results are obtained using the generally accepted aluminum recovery methods described heretofore.
__________________________________________________________________________ Pounds Metallics Recovered Metallics Recovered Pounds Pounds At Alloyed Recovered as % of to Contained Metallic Aluminum as % of Entrained Size Range Furnace Metallics % Ingot Dross Metallics __________________________________________________________________________ +1 inch 20,000 18,000 90% 16,200 81% 90% -1 inch +.5 inch 24,000 20,400 85% 16,320 68% 80% -.5 inch +.25 inch 26,000 20,800 80% 14,560 56% 70% -.25 inch +.10 inch 30,000 22,500 75% 11,250 38% 50% 100,000 81,700 82% 58,330 58% 71% __________________________________________________________________________
Therefore, using prior art methods of metallic recovery, substantial portions of the contained aluminum or aluminum alloy in the low grade, smaller dross particles are lost in the recovery process. This results in a metal recovery, by weight, of approximately 58% of the dross load or approximately 71% of the contained metallics.
It is to be further understood that conventional prior art milling methods could be employed to mill dross for an extended period of time with the resulting dross having an increased metallic % content. However, this is not done for at least two reasons. First, it would require significant amounts of energy to mill the dross for prolonged periods of time which is economically impractical. Secondly, if the dross were continuously milled for a prolonged period of time, the milling would tend to disintegrate some of the metal into dust which would combine with the oxides whereby such disintegrated metal could not be used to charge a furnace.
Therefore, with respect to the current state of the art, it will become readily apparent that the present invention represents a significant breakthrough in the processing of dross as hereinafter shown.
Accordingly, one object of the present invention is to provide a new and substantially improved process for recovering a larger portion of the entrained aluminum from dross which heretofore has been done with lesser effectiveness due to the difficulties in effectively processing the same.
Another object of the present invention is to provide a new and improved dross processing method whereby low percentage metallic content aluminum oxide dust may be recovered for use in various industrial applications.
A principal object of the present invention is to provide a method wherein aluminum concentrates of selected degree are mechanically recovered from partially prepared low grade dross concentrates.
Another object of the present invention is to produce very high percentage recoveries of aluminum entrained in dross, as aluminum ingot, using conventional prior art furnaces and the present invention.
A further object of the present invention is to recover aluminum entrained in dross at a lower cost of recovery than methods now in use by reducing the amount of fluxing materials needed and by maximizing energy and labor resources, which results in minimizing the energy and labor cost per pound of aluminum produced in the furnace, and by reducing waste disposal problems.
Still another object of the present invention is to provide a new dross processing method whereby high purity aluminum pellets may be recovered for use in various industrial applications.
Another object of the present invention is to reduce the amount of waste created and air pollution problems created in prior art recovery processes; thereby significantly reducing environmental problems.
Another object of the invention is to convert irregularly sized chunks of a metallic dross or similar material, having a predetermined size range into metallic pellets or powder comprising particles having a predetermined size range.
In summary, raw dross may initially be screened, milled, screened and separated into three fractions, for example, as familiar to those skilled in the art. The particles of these fractions may be one-quarter of an inch and larger, one-quarter of an inch down to one-tenth of an inch and one-tenth of an inch and down in size. This initial preparation and sizing is prior art and not part of the present invention.
Dross concentrates or particles larger than one inch, for example, which approximates 20% of a representative sample of partially prepared dross concentrates, are already high grade, high % metallic concentrates to a degree sufficient for final processing. The size ranges on the order to one inch and smaller are sub-divided and processed by the method and apparatus of the instant invention.
A sub-divided size range, low grade dross concentrate is screened and conveyed to a first pair of roller means having a predetermined spacing. The rollers are preferably spring or otherwise resiliently mounted to allow movement of the rollers away from one another as the dross passes therebetween. The rollers compress the dross particles or concentrates to a limited degree without substantially crushing the same so as to break the bonds between the metalic and non-metallic substances in the dross. The dross concentrates, so compressed, are screened to remove the limited amount of oxides which fall off as a result of the roller action. The screened dross, in the compressed condition, is then conveyed to a hammermill which loosens and removes the oxides from the aluminum in a highly effective manner due to the prior breaking of the bonds between the metallic and oxide substances by the rollers. The oxides are then screened out to yield aluminum concentrates significantly free of oxides. These resulting concentrates may be charged into a recovery furnace or conveyed to a second pair of roller means. The second pair of rollers substantially crush the high grade aluminum concentrates into flattened high grade aluminum flakes. A small percentage of oxide material is removed by the action of the second rollers.
At this point, the aluminum flakes may be further processed as follows:
1. The flakes may be charged into a furnace for recovery of the contained metal as aluminum ingot with recovery ratios being approximately the same as partially prepared dross concentrates sized larger than one inch in dimension.
2. The aluminum flakes may be processed through a series of hammermills to convert the same into high purity aluminum pellets sized on the order to one-tenth of an inch and smaller, for example. The larger shredded pieces tend to ball up into substantially pure aluminum pellets. The smaller shredded pieces also tend to ball up into aluminum pellets, but of lesser purity because they become intermixed with tiny pieces of oxide not removed by the process.
3. The substantially pure aluminum pellets may be charged into a furnace for recovery of the contained metal as aluminum ingot with recovery ratios better than the recovery ratio of partially prepared dross concentrates sized larger than one inch as referred to hereinabove.
The foregoing and other objects, advantages and characterizing features of the present invention will become clearly apparent from the ensuing detailed description and an illustrative embodiment thereof, taken together with the accompanying drawings wherein like referenced characters denote like parts throughout the various views.